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    Re: sextant index error measurement
    From: Frank Reed CT
    Date: 2006 Nov 4, 19:23 EST
    Bill you wrote:
    "On topic he did mention, if I recall correctly,
    that in some parts of the USA  traditional methods (optical?) had to take a
    break during the hottest part of the day due to refraction.  I failed to
    mention that as I trust it will affect both beams almost uniformly, and it
    is better to be thought an idiot than to open one's mouth and prove it."
     
    You're correct that you do not need to worry about refraction in this case, primarily because the distance is so short. Refraction will bend a laser beam downward by anywhere from one to several tenths of a minute of arc in one nautical mile of lateral distance. Over the few hundred feet of distance involved in this test, the deflection will always be less than a tenth of a minute of arc. And of course, the differential deflection of two beams side-by-side will be smaller again by an order of magnitude. So it's completely negligible.
     
    Just for fun, let's imagine an extreme exception... Take your sextant out to a parking lot with nice black asphalt surfacing on a calm summer day just after sunrise. Set it up vertically just above the surface and wait. As the Sun heats the surface a layer of hot air will form over the parking lot. Without wind, the hot layer will not immediately mix fully with the cooler air above it. The density gradient will be substantial, and the refraction will be measurably different. As the doctor says in the old joke, "Vell, don't do that!".
     
    -FER
    42.0N 87.7W, or 41.4N 72.1W.
    www.HistoricalAtlas.com/lunars

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